1. Field of the Invention
The present invention relates to a loudspeaker assembly in which a voice coil is driven by a magnetic field (hereinafter called repulsive magnetic field) generated by a magnetic circuit (hereinafter called repulsive magnetic circuit) having two magnets which are magnetized in the direction of thickness and the magnetic poles of the same polarity are faced with each other with a center plate being interposed therebetween.
2. Related Background Art
A conventional loudspeaker has the magnetic characteristics and the structure such as shown in FIGS. 5 and 6.
FIG. 5 explains the magnetic flux distribution and the directions of lines of magnetic force, respectively of a magnetic circuit of a repulsive magnetic type loudspeaker. FIG. 6 is a cross sectional view showing the structure of a conventional loudspeaker.
A repulsive magnetic circuit has been considered not suitable for driving a voice coil 1c of a loudspeaker of the type that the diaphragm is driven at a large vibration amplitude, because of the magnetic characteristics specific to this magnetic circuit. As seen from the magnetic flux distribution of the magnetic circuit shown in FIG. 5, magnetic fluxes are guided from the magnetic poles of the same polarity (N pole) facing each other into a center plate P interposed between the two magnets M1 and M2, extend from the outer periphery of the center plate P, and immediately flow toward the opposite magnetic poles (S pole).
Also in the case where an outer ring L or the like of magnetic material is disposed outside of the center plate P to form a magnetic gap therebetween, although a portion of magnetic fluxes extending from the outer periphery of the center plate P directly flows toward the opposite magnetic poles (S pole), most of the magnetic fluxes pass through the outer ring L and thereafter immediately flow toward the opposite magnetic poles (S pole).
Therefore, the magnetic flux distribution of this magnetic circuit provides a pattern as in the following. Namely, the magnetic flux density at the voice coil is great near the center plate P, decreases at the positions higher and lower than the position of the center plate P, becomes "0" at the position approximately 1/3 to 1/2 of the thickness of each magnet, increases at the positions higher and lower than the "0" point position with the direction of magnetic fluxes being reversed, takes a negative side maximum value at the positions corresponding to the top and bottom surfaces of the two magnets, and decreases toward "0" as the positions become higher and lower from the maximum positions.
The voice coil 1c vibrates in the upward and downward directions as viewed in FIG. 6 to drive a diaphragm 6 and produce sounds. As the voice coil 1c for driving a low frequency loudspeaker or woofer, a so-called long voice coil is used as shown in FIGS. 5 and 6 in order to improve the amplitude efficiency at the low frequency band. Generally, such a long voice coil 1c has a winding width about two to three times wider than the thickness of the center plate P or top plate T.
However, although sufficient magnetic fluxes cross the voice coil 1c near at the center plate P to drive it, the negative side magnetic fluxes suppressing the normal motion of the voice coil 1c are present at the positions higher and lower than the center plate P.
Specifically, the long voice coil 1c in the repulsive magnetic circuit receives the magnetic fluxes near at the center plate P at the central area of the coil. However, at the other area near at the top and bottom surfaces of the magnets, the voice coil 1c enters the negative side magnetic flux distribution region. Therefore, the total magnetic fluxes for driving the voice coil 1c decrease, lowering the sound pressure.
As described above, as a voice coil for driving a woofer, a long voice coil has been conventionally used in order to improve the amplitude efficiency at the low frequency band. Generally, such a long voice coil has a winding width about two to three times wider than the thickness of the top plate.
However, although sufficient magnetic fluxes cross the voice coil near at the center plate to drive it, the negative side (opposite direction) magnetic fluxes suppressing the normal motion of the voice coil are present at the positions higher and lower than the center plate.
Therefore, the long voice coil receives the magnetic fluxes near at the center plate P at the central area of the coil. However, at the other area near at the top and bottom surfaces of the magnets, the voice coil enters the negative side magnetic flux distribution region. Therefore, the total magnetic fluxes for driving the voice coil decrease, lowering the sound pressure.